Process for reducing chromium compounds



Patented Jan. 3, 1939 PROCESS FOR REDUCING CHROMIUM COMPOUNDSApplication August 3, 1937, Serial No. 157,154

17 Claims.

'I'his invention relates to a method of reducing a. chromium chloride tochromium, and, more particularly, to a method of reducing chromiumtrichloride with hydrogen to provide chromium and especially chromium assponge chromium.

A definition of sponge chromium The term sponge as applied to a metalhas a very denite meaning. The term has been herem tofore looselyapplied in the art as describing metals heretofore produced by lowtemperature operations without regard for the actual metal produced.Sponge metals have been properly dened as those reduced, usually bygaseous agents, from oxidic compounds of the metal under such lowtemperature conditions with respect to the melting point thatgrain'growth, cementation, or incipient fusion of the reduced metal doesnot take place. The material is thus macroscopically pseudomorphid withthe particles of the source material, but under high magnication is seento consist of aggregates of very minute metailic particles, which ingeneral are of an individual grain size of a micron or less. Since ,Dthe macroscopic form is preserved, the whole structure is thus cellularor porous, whence the name sponge metal is applied.

While sponge iron is so produced commercially,

and is a material recognized as having metallurgical advantages forspecial purposes, sponge chromium has not been produced commercially,nor have previous investigations disclosed a workable method ofmanufacturing it, despite its obvlous interest for use in powdermetallurgy.

The prior art The reason why sponge chromium can not be prepared byconventional methods, such as are lused for iron, resides, I have found,in the exceptionally high stability of chromium oxide (CrzOa). Usualreducing gases are only very weakly reducing on this oxide even at veryhigh temperatures. Thus, taking the typical example of the reduction ofCrzOs by hydrogen, the equilibrium constant for the reaction is at 1200C., 1.3 .l04; at 1400 C., 5.5X104; and near the melting point ofchromium (which has been given at 1530-1570 C.) about 1.6 103. Thus theamount of water vapor produced by passing pure hydrogen at oneatmosphere pressure over chromium oxide is at 1200 C., only .01%; at1400 C. about .06%, and at the melting point, about 0.16%. This showsthe high sta- 55 bility of this oxide.

It has been proposed to reduce chromlc oxide by hydrogen at temperaturesnear 1500 C., by recirculating hydrogen gas repeatedly (obviously over athousand cycles would be needed to use it up at 1500 0.), and each timeremoving the minute amount of water formed per cycle by freezing it outwith liquid air. Such a process I deem uneconomical, and subject togreat dimculty in adaptation to commercial use because of the veryserious diiiiculties of nding materials gas tight to hydrogen at thesetemperatures, as well as to the low cyclic efliciency.

Other investigators have produced what they term as sponge metals,`including chromium. However, upon consideration of these, it will bereadily seen that they do not have and could not possiblyhave producedsponge chromium. For example, Flodin in U. S. Patent 1,792,532 ofFebruary 17, 1931, produced an. alloy of iron and chromium. Flodin rstformed an ironchrome oxide briquet which contained silicon but 'which heclaimed was substantially free of carbon. Thereafter, this briquet washeated to 1150-1300" C. to the end that the silicon reduced the chromiumoxide. Since the material included iron, grain growth necessarilyfollowed, since this begins in iron near 900 C. It is obvious thatmodins material could not answer the definition of a substantially pure,true sponge chromium.

It has been proposed to reduce chromic oxide with hydrogen at 900-1100'C. according to the reaction See Rich, U. S. Patent '1,741,955. Meyerhas (Kaiser Wilhelm Inst. Eisenforch 13, 1931, p. 199) stated that hecould not elect this reduction at 900 C. In any event, assuming Rich hasan .operative process, the equilibrium constant for this reaction is1.3X10-4 at 1200 C. My experimental observations have shown grain growthin chromium to begin below this temperature and to be appreciable atthis temperature. From the low equilibrium constant it should be obviousthat even at 1200 C. an enormous hydrogen to oxide ratio must beused.Rohn, in Patent 1,915,243 of June 20, 1933, proposed operation at 2000C.

Baukloh and Henke (Zeit. fur Anorg. und All. Chemie 237 (1937),307-310), give the percentages of reduction effected by hydrogen forvarious temperatures. According to them about eight hours are requiredat 1000 C. to eiect as much asv an 8% reduction and at 1200 C. only 40%reductionisachievedintliistimeperlod.

The non-equivalence of hydrogen and other reductants As will appearhereinafter,.I have found that the reduction of chromium chloride toproduce sponge chromium must be conducted in a relatively narrowtemperature range-between 775- 815 C., the uppertemperature being thatat which chromous chloride melts. Preferably, -the reaction is carriedout between 775 and 802 C. In Patent 1,373,038, Weber states thatchromium chloride can be reduced by metallic iron to metallic chromiumover a temperature range between 700 and 1200 C. I have determined thatwhen the reducing agent is not a gaseous medium, a true sponge metalcannot result as a product.

In the case of Webers process, the reducing medium is a metal,particularly iron. Within the temperature range specified by Weber, ironis not a gaseous reducing medium. It should also be particularly notedthat the reduction of chromlum oxide at temperatures near its meltingpoint causes rapid grain growth in the metal, and such a product couldnot fall within the proper and accepted definition of a true spongemetal.

While carbonaceous reductants, such as carbon monoxide, or methane, aremore powerful reducing agents than hydrogen at temperatures above about900 C., these materials result in the formation of chromium carbide,which renders the product unsuited to the special requirements of powdermetallurgy or the manufacture of stainless steels. Hydrogen thereforestands alone as a reducing agent for production of sponge chromium froma chromium chloride.

General considerations of the process I have discovered a method ofreducing a chromium bearing material, as CrCl3, to metallic chromium ata temperature as low as 780-800 C. by hydrogen gas, and have succeededin manufacturing a new and desired metallurgical material, spongechromium of high purity.

My investigations of the reduction of chromous chloride by hydrogenCrCl2|H2=Cr+2HCl show that the amount of hydrochloric acid inequilibrium with hydrogen (when the total pressure was 1 atmosphere) wasat 700 C., 1.05%; at 750 C., 1.87%; at 800 C., 3.12%; and at 815 C.,3.60%. At 815 C., pure chromous chloride melts, and, in the process ofmelting, easy access of hydrogen is hindered, so that reduction is slowat temperatures in the liquid range; 815 C. at one atmosphere thereforeprovides a limiting upper temperature. V

The first stage of reduction of chromium trichloride is, under normalcircumstances, to the dichloride, rather than to metal. The reaction2CrCla-l-H2=2CrCl2+2HCl is substantially irreversible, and for allpractical purposes may be considered to go completely to the right, andat equilibrium results in virtually complete usage of the hydrogen. Thereaction rate becomes appreldrogen .to metal is possible with atheoretical utilization of hydrogen corresponding to the limit set byequilibrium of 3.12% HC1 at 800 C., and in a continuous counter-currentreduction apparatus the Hunting normal HClcontent is corresponding tothe stoichiometric relationships Cr:CrCl2:CrCl3. Any concentration ofHC1 above 3.12% will cause reversal in that part of the reduction unitwhere the metal is present, but by using a counter-current process I cansecure higher HC1 concentrations. To avoid fusion and volatilization,and to ensure the macroscopic retention of form required to producesponge chromium, temperatures above 802 C. should not be utilized,although where sponge chromium is not desired the temperature can varybetween 775 C. and 850 C. Above 850 C. the' material vaporizes fasterthan it reduces and diiculties due toV vaporization are encountered.

Hydrogen purification Passing commercial hydrogen over the trichlorideunder these conditions, did indeed produce somechromium metal, but onlyin a form badly contaminated with oxide. Further, I found that ordinarymethods of removing the hydrochloric acid gas from the reducing hydrogenby chemical absorption methods in water or basic solutions were entirelyunsuited to the regeneration of the reductant gas. The reason for thiswas traced to the fact that chromium chloride has a very low tolerancefor water vapor or any oxygen containing gas in fact. Thus, for example,I found that the reaction permits at 800 C. of a partial pressure of H2Oof only 1.5 10*6 atmospheres when the pressure of HC1 is 1 atmosphere,but under conditions permitting the continuous reduction of CrClz,corresponding to 3.12% HC1, the tolerance drops and is only (1.5X10-6)(0.0312)2=1.5 10f9(approx.) as the partial pressure of water vapor inatmospheres. Any amount of water vapor in excess of this is completelyreacted to chromium trioxide; Similarly, I found the tolerance of CrCla,corresponding to the reaction is a partial pressure of water vapor of4.5X10-5 atmospheres, vat 800 C., when the partial pressure ofhydrochloric acid is 0.0312 atm. and hydrogen .is practically 1atmosphere.

Finally; in the oase of chromium metal itself, I found th'e tolerancecorresponding to the reaction 2Cr+3H2O=Cr2Oa+3H2 is 2.7 10'I atmospherespartial pressure of water vapor at 800 C. when the hydrogen is near oneatmosphere.

From the above it is clear that the impurities in commercial hydrogen,including oxygen from a sol Manner of practicing the invention I havediscovered a presently related method of eliminating these diillcultiessuccessfully with respect to hydrogen and of setting up a continuousreductionand purication which produces substantially chemically puresponge chromium which contains less than about 0.5% of oxide, withtraces only of iron and carbon.

The manner in which the'invention is preferably practiced will be bestdisclosed and understood by reference to the drawing, wherein the singlefigure is a flow sheet indicating in a schematic manner the process ofthe present invention.

Commercial hydrogen is first purified of oxygen as by passing it througha drier 3, then over a nickel catalyst at about 600 C., and again over adrier 5, and thence passes into storage container 6, where it is storedover a dehydrating agent as concentrated sulphuric acid. (Suitablevalves, pressure regulating devices, temperature and pressure indicatingdevices, are included in the operating system but are not shown.)

The hydrogen storage container 6 is connected to a closed gascirculating system consisting of a pair of sorption purifiers l and 8containing active carbon, a circulating pump 9, a final purifier Icontaining active carbon, and the reducing furnace Il. The firstsorption purifiers l and 8 are operated at ordinary temperatures,alternately. It has been found that commercial grades of active carbonare capable of preferentially absorbing hydrochloric acid gas up to 4 to5% of the weight of carbon under these conditions, before appreciableHC1 passes through an absorber unadsorbed. When this point is reached.the alternate absorber is connected in the circulating system, and thesaturated one is desorbed by first heating to about 200 C., Which drivesoff approximately 80% of the retained HC1, after which the temperatureis further raised to about 300 C., and the purifier is flushed withabout of the volume of gas which has been purified during the previoushalf cycle, using fresh hydrogen, admitted through line I2 from thestorage container 6. This removes the remainder of the adsorbed HC1 andwater vapor. It is then ready for further use. The HC1 is driven off andabsorbed in water or any desired material suitable for the preparationof a commercial by-product in the absorber After leaving the pump, andbefore actually entering the reduction furnace, the gas is given a finalpurification in adsorber I0 containing active carbon and cooled to -70to -40 with carbon dioxide snow. Small amounts of water vapor passingthe larger absorbers (which are essentially the HCl adsorbers) are thusvirtually completely removed. The regeneration of the accessory purifierneed be accomplished only after relatively long periods of operation,since the amount of material retained is small. The regeneration iscarried out in a manner similar to the larger` units.

The reduction unit is an electric tube furnace il of square orrectangular cross section through which trays containing CrCla crystalsare passed countercurrent to the flow of hydrogen gas, the maximumltemperature being 802 C. Thegases in the furnace are not corrosive toordinary steel, and the tube of the furnace may be either steelprotected on the outside from oxidation by air, or of a non-scalingchrome-nickel steel. An especially satisfactory type of construction isto provide a double walled tube, the outer wall 2| being chrome nickelsteel, and the inner wall 22 of ordinary low carbon steel. Hydrogen fromline 23 is admitted to the space between the walls and is maintained atslightly above l atmosphere pressure. This permits the use of either asingle, or multiple assembly of inner ducts, which may be made of lowcarbon steel of light gauge, and

which are readily replaceable. This construction Y gas, suitable gaslocks 26 and. 21 are placed at the inlet and outlet end of the furnace.'I'hese are operated in the usual manner, being purged and filled withhydrogen from 4lines 28 and 29 to ensure preservation of the oxygen freeatmosphere in the furnace.

I have found that, while normally the reduction takes place in steps,flrst to CrCla and later to metal, resulting in concentrations near 4.7%HC1 maximum when using the continuous countercurrent apparatus, that, byusing high gas velocities and causing the trays to enter rapidly the800. zone of the furnace, I am able to obtain an exit gas as high as 8to 10% HCl. This may be interpreted as showing that to some extentdirect reduction of CrCla to metal is in part attained. I have alsoobtained hydrochloric acid concentrations above the CrCla-Hz-Cr-HCllimit when using slow gas rates and slow rates of tray admission, but inthis case the result was presumably due to a preponderance of thereaction.

My invention isv applicable to the reduction of either of the solidchromium chlorides, whether stepwise or direct, and may obviously alsobe utilized for producing anhydrous chromium chloride, since theformation of metallic chromium can be` completely prevented at low gasrates by maintaining the entrant hydrogen with a content of HC1 of atleast 3.12%.

In practicing my invention, it is desirable to remove all' traces ofmoisture which may be contained on or in the chromium chloride. Thesocalled violet anhydrous crystals of CrCla, which are virtuallyinsoluble in water, nevertheless are capable of adsorbing severalpercent water when they have previously contacted this medium as liquid,or been stored in air of normal humidity. I have found it desirable,therefore, to dry the crystals by heating them, preferably to 200-300 C.and preferably in a vacuum, before introducing them into the reductionfurnace, and to carry out the transfer quickly, or in a driedatmosphere, sothat recontamination is avoided. Temperatures higher thanthis must not be used not only because the chloride vaporizes anddissociates, but also because the crystals become reactive with oxygenat temperatures slightly above 300 C. This step is indicated at 3 l Thusit is seen that While one patentee found it necessary to utilizetemperatures above 1600 C.

to reduce chromium successfully by hydrogen, by g5 carrying on thereduction from the CrCla form under the conditions, and in the mannerspecied above, I have been able to produce chromium in the true spongeform at temperatures of 800 C. and in a continuous counter-current unit,and have demonstrated the feasibility of producing a.

'new form of chromium having metallurgical advantages. The new product,sponge chromium, is claimed in a companion application, Serial No.157,153 of even date.

I claim:

l. A process for the reduction of a chromium chloride to chromium'comprising reducing said chloride with substantially only dry hydrogenat a temperature of about 775 C. in a reducing zone.

2. A process for reduction of a chromium chloride to chromiumcomprising'reducing said chloride with substantially dry pure hydrogenlat a temperature of 775-850 C. in a reducing zone, removing hydrogencontaining hydrogen chloride from the reducing zone and separatinghydrogen chloride and water vapor therefrom by a treatment includingpassage of the hydrogen over activated carbon, and returning the puriiedhydrogen to the reducing zone.

3. A process for the reduction of a chromium chloride to chromiumcomprising reducing said chloride with substantially dry hydrogen atatemperature of about 775 C. in a reducing zone, removing hydrogencontaining HC1 from the reducing zone, separating HCl and water Vaporfrom said removed hydrogen by contacting said removed hydrogen withactivated carbon, and returning the purified hydrogen to the reducingzone.

4. In a process of reducing a chromium chlov ride yto chromium, the stepof passing substantially dry pure hydrogen counter-current to saidchloride in a reduction zone in a furnace at about 800 C. at such a ratethat hydrogen leaving said l furnace carries about 4.7% HCl.

5. In a process of reducing chromium trichloride to chromium dichloride,the step of passing substantially dry oxygen free hydrogen containingabout 3.12% HCl over said trichloride chloride in a reduction zone at atemperature of about 802 C.

6. In a process of reducing chromium trichloride to chromium dichloride,the step of passing substantially dry oxygen free hydrogen containingvdirect contact therewith substantially dry pure hydrogen. I

9. A process for the production of sponge chromium comprising heatingsubstantially dry chrobetween 775 and 815 mium trichloride to atemperature of 775-802 C. and passing dry pure hydrogen thereover toreduce said chloride to a chormium macroscopically pseudomorphic withsaid chloride and substantially free of grain growth.

10. Reducing chromic chloride to chrome by passing dry hydrogen over amass consisting of substantially only said chloride at a temperaturebetween 775 and 802 C. to reduce said chromic chloride directly tochromium, and continuing to pass said hydrogen until said mass .consistssubstantially only of chrome.

11. Reducing chromic chloride to chrome by passing dry hydrogen over amass consisting of substantially only said chloride at a temperature C.to reduce said chromic, chloride directly to chromium, and continuing'to pass said hydrogen until said mass consists substantially only ofchrome.

12. Reducing chromic chloride to chrome by passing dry hydrogen over amass consisting of substantially only said chloride at a temperaturebetween 775 and 850 C. to reduce said chromic chloride directly tochromium, and continuing to pass said hydrogen until said mass consistssubstantially only of chrome.

13. Reducing chromic chloride to chrome by passing dry hydrogencontaining less than 4.7% HCl over a mass consisting of substantiallyonly said chloride at a temperature between 775 and 802 C. to reducesaid chromic chloride directly to chromium, and continuing to pass saidhydrogen until said mass consists substantially only of chrome.

14. Reducing chromic chloride to chrome by passing dry hydrogencontaining less than 4.7% HCl over a mass consisting of substantiallyonly' said chloride at a temperature between 775 and 815 C. to reducesaid chromic chloride directly to chromium, and continuing to pass saidhydrogen until said mass consists substantially only of chrome.

15. Reducing chromic chloride to chrome by passing dryvhydrogencontaining less than 4.7% HC1 over a mass consisting of substantiallyonly said chloride at a temperature between 775 and 850 C. to reducesaid chromic chloride directly to chromium, and continuing to pass saidhydrogen until said mass consists substantially only of chrome.

16. In a process of reducing chromium trichloride with hydrogen tochrome metal whereby a hydrogen eilluent stream containing about 5%hydrogen chloride is produced, the steps of passing said eilluent streaminto contact with activated carbon to remove said hydrogen chloridesubstantially entirely and simultaneously maintain the water contentthereof at a point whereat the partial pressure of water vapor in thehydrogen is less than about 4;5 10-5 atmospheres, at a temperature ofabout 800 C.

17. In a process of reducing chromium trichloride tochrome metalutilizing hydrogen as the vreduc-tant, the steps which consist inemploying as feed material chromium trichloride containing less than0.05% wat-er while maintaining the reduction temperature between 775 and850 C. CHARLES G. MAIER.

